In ColumnSetFrame's reflow methods, mCBReflowInput is equal to
mParentReflowInput in most of the cases.
However, a multicol <button> has the HTMLButtonControl as the outermost
frame, where ColumnSetWrapper is its -moz-button-content anonymous
child. In this case, mCBReflowInput is HTMLButtonControl's reflow input.
To get the correct computedBSize of ColumnSetWrapper, we need to use
mParentReflowInput.
Differential Revision: https://phabricator.services.mozilla.com/D41497
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extra : moz-landing-system : lando
